This invention relates to a DC resistance welding machine incorporating an invertor and, more particularly to a control system for such welding machine assuring a high and reliable quality weld.
Recently, DC resistance welding machines that utilize an invertor in its power circuit referred hereinafter as invertor type welding machine have been developed and are spreading in the market.
As compared with conventional single phase AC resistance welding machine, the invertor type DC resistance welding machine has the following advantages.
First, it is possible to reduce the size of a welding transformer incorporated in the invertor type welding machine since a high-frequency power is supplied to the transformer. In applying the welding machine to a welding robot, such small sized transformer can be mounted on an end portion of an arm of the robot with no cable or electrical wire being required between the secondary coil of the transformer and a welding gun, thereby saving both cost and power consumption.
Secondly, DC welding by the invertor type welding machine is of higher heating efficiency, and requires a smaller weld current flow or conduction in workpieces as compared with AC welding by the AC welding machine, thus, it is economical in power consumption and the life of electrodes can be elongated.
Thirdly, commercial three phase AC can be utilized as the power source of the invertor type welding machine, in which three phase balanced load and high power factor are obtained.
Heretofore proposed invertor type DC welding machines generally incorporate a constant-current controller or timer which functions to maintain constant the RMS value of weld current flowing during a welding operation. The controller or the timer is based on the idea of a constant-current phase control system of the single phase AC type welding machine and the weld current is deemed as the principal requirement to obtain stable and high quality weld.
However, the weld current is DC in the invertor type welding machine while AC in the single phase AC type welding machine, and when such DC weld current is maintained constant, there arise some disadvantages. When a pressure applied on electrodes is low or insufficient, for example, the portions to be welded of the workpieces being located between electrodes are in a high resistance conditions. In maintaining the weld current constant under such condition, it sometimes makes the resistance heat generation excessive which may cause splashing or explosion of the molten workpieces. Furthermore, when the weld current is maintained constant even though the contact area between the electrodes and the workpieces has become increased due to wear or abrasion of the tips of electrodes, it sometimes results in that the current density in the portion to be welded is too low to generate a sufficient quantity of resistance heat for obtaining a good weld bond.
Moreover, it is difficult to terminate a relatively short duration of welding operation, particularly for workpieces such as small metal assemblies or thin sheet of metal, at a suitable timing after a nugget is produced in the portion of the workpieces to be welded. In other words, premature termination may lead to shortage of weld energy while delayed termination to splashing which seriously affects the weld quality of such small workpieces.